This invention is related to optical imaging and metrology of semiconductor, data-storage, and biological materials, structures, and devices.
Practical optical data-retrieval devices employing moving media rely upon efficient and accurate detection of optical inhomogeneities representing patterns of binary bits. These bits may be encoded, for example, in optically discernable variations of topography, reflectivity, absorption or transmission.
The invention features systems and methods for near-field, interferometric microscopy in which a differential detection technique is used to investigate the profile of a sample, to read optical date from a sample, and/or write optical date to a sample. The systems may operate in either reflective or transmissive modes.
In general, in one aspect, the invention features an interferometric optical microscopy system for imaging an object. The system includes: (i) a measurement beam mask array having an array of aperture pairs positioned to receive radiation emitted from the object in response to a measurement beam, radiation emerging from the array of aperture pairs defining a measurement return beam; (ii) a reference beam source array positioned to receive a reference beam, the reference beam source array including an array of elements each configured to radiate a portion of the reference beam, the radiated reference beam portions defining a reference return beam; and (iii) imaging optics positioned to direct the measurement and reference return beams to the photo-detector and configured to produce overlapping conjugate images of the array of reference elements and the array of apertures pairs. The conjugate image for each aperture pair overlaps with the conjugate image of a corresponding reference element. Furthermore, the imaging optics include a pinhole array positioned in the conjugate image plane, the pinhole array having an array of pinholes each aligned with a corresponding aperture pair image. Finally, the measurement and reference beams are derived from a common source.
Embodiments of the system may include any of the following features.
Each pinhole in the pinhole array may be sized to pass only a central portion of each corresponding aperture pair image.
The system may further include a source for the measurement and reference beams. The source may be configured to direct the measurement beam to the measurement beam mask array, and each aperture in the measurement beam mask array is configured to radiate a portion of the measurement to the object to cause the object to emit the radiation. The measurement beam may contact the mask array at normal incidence. Alternatively, the source directs the measurement beam to contact the measurement mask array at an angle to a normal to the mask array, thereby introducing a phase shift between the measurement beam portions radiated to the object by the apertures in each aperture pair. Furthermore, the system may be implemented in a transmissive mode, in which case the measurement beam mask array is used only to collect radiation emitted from the object, and the system further includes a measurement beam source array positioned to receive the measurement beam. The measurement beam source array has an array of source aperture pairs positioned to radiate portions of the measurement beam to the object to cause the object to emit the radiation. The measurement beam may contact the measurement beam source array at normal incidence. Alternatively, the source directs the measurement beam to contact the measurement beam source array at an angle to a normal to the mask array, thereby introducing a phase shift between the measurement beam portions radiated to the object by the apertures in each aperture source pair.
The system may further include a multi-element photo-detector positioned to measure the radiation emerging through each pinhole. The radiation emerging through each pinhole provides an interference signal indicative of a differential property between object locations corresponding to the apertures in each aperture pair. The system may further include an electronic controller coupled to the photo-detector and configured to resolve the differential property across multiple regions of the object based on signals from the photo-detector.
In general, in another aspect, the invention features a differential microscopy system for imaging an object. The system includes a mask including an array of aperture pairs, each aperture pair having a common separation and an imaging system. During operation the mask is positioned adjacent the object to receive radiation emitted from the object. The imaging system is configured to image radiation emerging from the array of aperture pairs to produce a first conjugate image of the emerging radiation and an overlapping, second conjugate image of the emerging radiation laterally displaced relative to the first conjugate image by an amount corresponding to the aperture pair separation and a magnification of the imaging system. A superposition of the first and second conjugate images define a set of aperture pair images each corresponding to a different one of the aperture pairs. The superposition suppresses a contribution to each aperture image of a selected component of the radiation emerging from each corresponding aperture pair.
Embodiments of the system may include any of the following features.
The selected component may be an anti-symmetric component of the radiation emerging from each corresponding aperture pair.
The imaging system may be further configured to impart a selected phase shift between the first and second conjugate images, and the selected component corresponds to the selected phase shift. For example, when the selected phase shift is xcfx80 (modulo 2xcfx80), the selected component is a symmetric component of the radiation emerging from each corresponding aperture pair. Additional values of the phase shift will cause the selected component to be a superposition of symmetric and anti-symmetric components of the radiation emerging from each corresponding aperture pair.
The imaging system may include an interferometer for separating and recombining the radiation emerging through the multiple sets of aperture pairs into portions that produce the first and second conjugate images. The imaging system may further include two collimating lenses defining a microscope and the interferometer may be positioned within the microscope. The interferometer may be configured to recombine the portions that produce the first and second conjugate images within the microscope and introduce a difference in propagation directions between the recombined portions. In such a case, the difference in propagation directions produces the lateral displacement between the first and second conjugate images. The interferometer may further be configured to introduce a relative phase shift between the recombined portions, and wherein the selected component is a superposition of symmetric and anti-symmetric components, the superposition being based on the relative phase shift.
Alternatively, for example, the imaging system may include a prism positioned at a pupil plane of the imaging system. The prism is positioned to contact a first portion of the imaged radiation, and not a second portion of the imaged radiation. The prism introduces a difference in propagation between the first and second portions to produce the laterally displaced first and second conjugate images. The imaging system may include two collimating lenses defining a microscope and the pupil plane may be positioned within the microscope. The prism may be further configured to introduce a relative phase shift between the first and second portions to cause the selected component to be a superposition of symmetric and anti-symmetric components.
The imaging system may further includes a pinhole array positioned in the conjugate image plane, the pinhole array having an array of pinholes each aligned with a corresponding aperture pair image. Each pinhole in the pinhole array may be sized to pass only a central portion of each corresponding aperture pair image.
In general, in another aspect, the invention features a differential microscopy system for imaging an object. The system includes a mask including an array of aperture pairs and an imaging system. During operation the mask is positioned adjacent the object to receive radiation emitted from the object. The imaging system is configured to image radiation emerging from the multiple sets of aperture pairs and produce a conjugate image of the emerging radiation. The conjugate image includes an array of aperture pair images each corresponding to a different one of the aperture pairs. The imaging system includes a pinhole array positioned in the conjugate image plane, the pinhole array having an array of pinholes each aligned with a corresponding aperture pair image. The imaging system furthers includes a wave front modifier positioned in a pupil plane of the imaging system. The wave front modifier causes the conjugate image to suppress a selected component of the radiation emerging from each aperture pair from passing through the corresponding pinhole in the conjugate image plane.
Embodiments of the system may further include any of the following features.
Each pinhole in the pinhole array may be sized to pass only a central portion of each corresponding aperture pair image.
The selected component may be a symmetric component of the radiation emerging from each corresponding aperture pair. Furthermore, the selected component may be a superposition of symmetric and anti-symmetric components of the radiation emerging from each corresponding aperture pair.
The wave front modifier may be a phase mask that imparts one or more phase shifts to different regions of the incident radiation. The phase mask may impart a constant phase shift to the different regions of the incident radiation. For example, the phase mask may impart a phase shift of xcfx80 (modulo 2xcfx80) to half of the incident radiation relative to the other half of the incident radiation. As a result, the selected component may be a symmetric component of the radiation emerging from each corresponding aperture pair. In one particular example, the phase mask may be defined by a retardation plate positioned to bisect the radiation in the pupil plane, wherein the retardation plate has a thickness that imparts a constant xcfx80 (modulo 2xcfx80) phase-shift relative to a similar thickness of air. Alternatively, the phase mask may impart a phase shift of xcfx80 (modulo 2xcfx80) to alternating periodic regions of the incident radiation. Furthermore, in additional embodiments, the phase mask may impart a linear phase ramp to one ore more selected portions of the incident radiation. Such a phase mask may be defined by a prism positioned to contact one-half of the radiation in the pupil plane and not the other half of the radiation in the pupil plane.
The imaging system may include two collimating lenses defining a microscope and the pupil plane may be positioned within the microscope.
In additional aspects, the invention features microscopy methods corresponding to the systems described above.
Confocal and near-field confocal, microscopy systems are also described in the following, commonly-owed provisional applications: Ser. No. 09/631,230 filed Aug. 2, 2000 by Henry A. Hill entitled xe2x80x9cScanning Interferometric Near-Field Confocal Microscopy,xe2x80x9d and the corresponding PCT Publication WO 01/09662 A2 published Feb. 8, 2001; Provisional Application Serial No. 60/221,019 filed Jul. 27, 2000 by Henry A. Hill and Kyle B. Ferrio entitled xe2x80x9cMultiple-Source Arrays For Confocal And Near-Field Microscopyxe2x80x9d and the corresponding Utility application Ser. No. 09/917,402 having the same title filed on Jul. 27, 2001; Provisional Application Serial No. 60/221,086 filed Jul. 27, 2000 by Henry A. Hill entitled xe2x80x9cScanning Interferometric Near-Field Confocal Microscopy with Background Amplitude Reduction and Compensationxe2x80x9d and the corresponding Utility application Ser. No. 09/917,399 having the same title filed on Jul. 27, 2001; Provisional Application Serial No. 60/221,091 filed Jul. 27, 2000 by Henry A. Hill entitled xe2x80x9cMultiple-Source Arrays with Optical Transmission Enhanced by Resonant Cavities and the corresponding Utility application Ser. No. 09/917,400 having the same title filed on Jul. 27, 2001; and Provisional Application Serial No. 60,221,086 filed Jul. 27, 2000 by Henry A. Hill entitled xe2x80x9cControl of Position and Orientation of Sub-Wavelength Aperture Array in Near-Field Microscopyxe2x80x9d and the corresponding Utility application Ser. No. 09/917,401 having the same title filed on Jul. 27, 2001; the contents of each of the preceding applications being incorporated herein by reference. Aspects and features disclosed in the preceding provisional applications may be incorporated into the embodiments described in the present application.
In preferred embodiments, the near-field scanning probe is typically a sub-wavelength aperture positioned in close proximity to a sample; in this way, sub-wavelength spatial resolution in the object-plane is obtained. An aperture smaller than a free space optical wavelength of an optical beam used in a near-field microscopy application is hereinafter referred to as a sub-wavelength aperture.
Embodiments of the invention may have any of the following advantages.
One advantage is sub-wavelength spatial resolution of a quasi-two-dimensional sample, e.g. an optical data-storage medium.
Another advantage is phase-sensitive detection of the complex scattering amplitude of a quasi-two-dimensional sample.
Another advantage is efficient optical throughput achieved by the use of a waveguiding near-field probe which supports two propagating modes at the operating optical wavelength.
Another advantage is detection of small changes in optical properties of a sample, using a sensitive differencing modality of the present invention.
Another advantage is spatially parallel and simultaneous operation achieved by the use of a plurality of wave guiding near-field probes, each of which supports two propagating modes at the operating optical wavelength.
Another advantage is a high degree of insensitivity to background scattered light.
Another advantage is an higher signal-to-noise ratio than may be obtained with a non-interferometric confocal microscope.
Another advantage is insensitivity to variations in total optical intensity and other environmental conditions external to a sample.
Another advantage is a larger measurement bandwidth and faster scanning than may be obtained with a non-interferometric scanning confocal microscope.
Another advantage is operation with low-contrast samples.
Another advantage is increased data-storage density in high-contrast media, by application of counting systems of radix larger than two.
Another advantage is a discrete-time method for making a determination of both the real and imaginary components of complex scattering amplitudes.
Another advantage is a discrete-time method for making a determination of both the magnitude and phase of complex scattering amplitudes.
Other aspects, features, and advantages follow.